build-libssp: Stop building the real libssp from GCC

As mingw-w64 now provides these routines, we no longer need to build
the original library from GCC.

As building with -fstack-protector-strong still links with -lssp
(and -lssp_nonshared), we still need to provide libraries with these
names, but they can be empty dummy libraries.

.gitignore

@@ -1,6 +1,5 @@
 /llvm-project
 /mingw-w64
-/libssp
 /lldb-mi
 /make-*
 /libffi